This invention relates to optical polarizers and, more particularly, to polarizers which exploit the difference in coupling between TE and TM modes.
A polarizer, as defined by E. B. Brown, Modern Optics, Reinhold Pub. Corp., New York, 1965, page 51, is a device or material which exhibits preferential properties with respect to the degree of polarization of a beam of incident light. Polarizers are classified as dichroic polarizers, which show preferential absorption for light polarized in specified directions; birefringence polarizers, which have different refractive indices for light polarized in orthogonal planes; and reflection or transmission polarizers, in which the coefficient of reflection or transmission depends on polarization.
In optical communications systems, polarizers can be combined, for example, with polarization rotation modulators, quarter-wave plates, analyzers and the like to effect various functions such as amplitude or frequency modulation of a laser beam or optical isolation to prevent reflected light from creating instabilities in a laser.
One type of dichroic polarizer discriminates between TE and TM modes by increasing by several orders of magnitude the loss experienced by TM modes while simultaneously the loss for TE modes is only slightly increased. A multilayered structure for accomplishing this type of discrimination was analyzed theoretically by Y. Yamamoto et al, IEEE Journal of Quantum Electronics, Vol. QE-11, page 729 (1975). This device consisted of a thin buffer film of material with a low optical dielectric constant inserted between a waveguide layer and a metal cladding. As shown in FIG. 3 of their paper, the Yamamoto et al calculations predict that at a critical thickness of the buffer film, the attenuation of certain TM modes peaks whereas that of TE modes actually decreases.
In this type of polarizer, the discrimination between modes is enhanced by increasing its length along the direction of light propagation. However, in certain applications, most notably integrated optics, where device size is limited by considerations such as, for example, chip size and packing density, it may not be possible or desirable to employ this simple expedient to increase discrimination.